This invention relates generally to compositions and methods for the formation of protective, corrosion-inhibiting pigments without the use of chromium in the hexavalent oxidation state. More particularly, this invention relates to non-toxic, corrosion-inhibiting pigments based on tetravalent cerium, praseodymium, or terbium and methods of making and using the same.
Inhibiting the initiation, growth, and extent of corrosion is a significant part of component and systems design for the successful long-term use of metal objects. Uniform physical performance and safety margins of a part, a component, or an entire system can be compromised by corrosion. Magnesium, aluminum, zinc, iron, titanium and their alloys tend to corrode rapidly in the presence of water due to their low oxidation-reduction (redox) potentials. The high strength 2000 and 7000 series of aluminum alloys are used extensively in aircraft and are very sensitive to corrosive attack. Materials such as steels and carbon fibers with higher redox potentials will form a galvanic couple in water and promote corrosive attack when located near light metal alloys such as aluminum.
A bare metal surface or one that has been conversion coated, phosphated, sealed, rinsed, or otherwise treated will be protected by the application of a primer paint with a corrosion-inhibiting pigment. As used herein, the term “pigment” means chemically active compounds with the ability to inhibit corrosion at a distance, rather than simple colorants or opacifiers. Oxidative compounds that are effective as corrosion inhibitors tend to be highly colored and/or opaque. An effective corrosion-inhibiting pigment has throwing power and can protect exposed base metal in a scratch or flaw by oxidizing and passivating it at a distance during aqueous corrosion when dispersed in a suitable carrier phase. These compounds are usually solids or liquids that are typically dispersed in a liquid carrier or binder system such as a paint or wash. These compounds may also be used to help inhibit corrosion without a significant liquid carrier using an integral binder and/or a low-volatile application method. Barrier layer formers such as sol-gel coatings or polymeric films are also used, but they tend to have no inherent oxidizing character and no appreciable throwing power, and fail to protect the metal surface when the film is breached.
Pigments that contain hexavalent chromium (CrVI) compounds are the de facto standard for high-performance corrosion-inhibiting paints and coatings for metal protection and are a typical corrosion inhibitor used to protect aluminum, zinc, magnesium, iron, titanium, copper and their alloys. Zinc (C.I. Pigment Yellow 36) and strontium (C.I. Pigment Yellow 32) chromate pigments are typically used, although higher-solubility calcium and magnesium chromates have also seen some limited use as pigments. The coating vehicles of these pigments include alkyd-type primers, acrylic primers, and elastomeric sealants, among others. Some transition metal chromate pigments (e.g., complexed with copper, iron, manganese, or cobalt) and organic chromate pigments (e.g., bound with nitrogenous compounds such as guanidinium) have been used in protective coating systems. Barium or lead chromates have also been used as corrosion inhibitors, as well as colorants. Variations in chromate speciation (i.e., what the chromate ions are bound to) will result in significant differences in protection when used as corrosion-inhibiting pigments, due to differences in chromate solubility.
A clear correlation between performance and solubility of chromate pigments has been shown. However, oxidizing chromates can be dangerous to use as corrosion inhibitors if they are not delivered in sufficient quantity in a timely manner to the location of a coating breach. The chromate composition was far more important to the corrosion-inhibiting performance of the primer film than the organic coating composition.
A principle use of zinc and strontium chromate pigments is in wash- or etch-primer formulations for aluminum protection. Wash- or etch-primers, which have been used since the 1940s, represent one of the harshest application conditions for chromate pigments. Wash-primers are applied to metal surfaces under acidic conditions where the primer is cured as a corrosion-inhibiting film. Chromate pigment powders dispersed in an alcohol/resin base mixture are combined with an aqueous phosphoric acid diluent solution. The acid roughens the metal surface and initiates cross-linking of the resin to form a pigment-filled polymeric film. The chromate pigment may also be dispersed in other carriers that are not as harsh as the wash primer. However, if a corrosion-inhibiting pigment can survive the harsh conditions of acid diluent, then it can usually be successfully incorporated within other paint, polymeric, or barrier film systems for corrosion inhibition.
An important use of chromate pigments is in coil coating formulations for steel, zinc-coated steel, or aluminum sheet stock. Coil coatings can represent a challenging application environment for pigments in that cure temperatures for these paints can exceed 100° C. Corrosion-inhibiting pigments for these applications must exhibit both throwing power to inhibit corrosion and be thermally stable at elevated temperatures when incorporated into the paint.
The use of hexavalent chromium pigments allows a broad range of service conditions to be addressed by tailoring the solubility of the pigment to the application needs. Significant efforts have been made in government and industry to replace CrVI with other metals for corrosion-inhibiting applications due to toxicity, environmental, and regulatory considerations. An effective replacement for hexavalent chromate pigment needs to have throwing power for self-healing coating breeches. “Throwing power” is the ability of a highly oxidized ion, such as hexavalent chromium, to oxidize and passivate the exposed bare metal in a small scratch or flaw.
A number of materials have been introduced as corrosion-inhibiting replacement pigments for hexavalent chromium-based compounds. Commercially available corrosion-inhibiting pigments including compounds such as molybdates, phosphates, silicates, cyanamides, and borates, which have no inherent oxidizing character, have been used as alternatives to chromate pigments. Coatings that contain these materials can effectively inhibit corrosion as barrier films until the coating is breached, as by a scratch or other flaw. Films or coatings that do not contain oxidizing species can actually enhance corrosion on a surface after failure due to the effects of crevice corrosion.
Cerium is one non-toxic, non-regulated metal that has been considered as a chromium replacement. Cerium (like chromium) exhibits more than one oxidation state (Ce+3 and Ce+4). In addition, the oxidation-reduction potential is comparable to that of CrVI in acidic solutions. For example, in acid solution:Ce+4+e−Ce+3+1.72 VCr+6+3e−Cr+3+1.36 V
The CeIV ion is a very good oxidizing species with an oxidation-reduction potential of +1.72 V (at pH 0). The hydroxyl and oxygen liberated from water when CeIV is reduced will oxidize nearby bare metal. This results in a passivated metal surface if sufficient oxygen is released. The potential required to reduce tetravalent cerium to trivalent cerium is only 0.36 volts greater than that needed to add three electrons to reduce CrVI to trivalent chromium (CrIII). CeIII is formed during corrosion inhibition by the oxidation of base metal in the presence of CeIV and water. CeIII is similar to CrIII in that neither is particularly effective as a redox-based corrosion inhibitor.
Praseodymium and terbium also exhibit more than one oxidation state (Pr+3 and Pr+4; Tb+3 and Th+4). Tetravalent praseodymium and terbium are even stronger oxidizing agents than cerium (with calculated redox potentials of +3.2 V in acidic solution (Nugent, L. J., et al., J. Inorg. Nucl. Chem. 33: 2503-2530, 1971):Pr+4+e−Pr+3+3.2 VTb+4+e−Tb+3+3.2 VCr+6+3e−Cr+3+1.36 VThe potential required to reduce PrIV to PrIII or TbIV to TbIII is over 1.8 volts greater than that to add three electrons to reduce CrVI to trivalent chromium (CrIII). PrIII or TbIII are formed during corrosion inhibition by the oxidation of base metal in the presence of PrIV or TbIV and water. PrIII and TbIII are similar to CrIII in that neither is particularly effective as a redox-based corrosion inhibitor.
A number of pigments using cerium, terbium, and/or praseodymium have been reported in the literature, but none approach the general performance or utility of CrVI-based pigments. Tetravalent cerium oxide (CeO2) and hydrated oxide pigments have been disclosed for corrosion protective coatings. However, the coatings formed provide only limited protection and do not approach the benefit derived from the use of hexavalent chromium.
A number of compounds have been described as cerium-, terbium-, or praseodymium-containing colorants or corrosion-inhibiting agents, including azo dyes, amidosulfonic acid derivatives, triazinedithiols, and triazinetrithiols. However, the rare earth is always described as being in the trivalent charge state. The pigments formed from these compounds provide only limited corrosion protection and do not approach the benefit derived from the use of hexavalent chromium.
Examples of the use of tetravalent cerium oxide pigments include Netherlands Application No. 6,601,070 to Associated Lead Manufacturers Ltd. (see Chemical Abstracts, vol. 65, col. 18303 (1966) (Abstract)) and U.S. Pat. No. 2,763,569 to Bradstreet et al. In addition, combinations of cerium oxides with transition or alkaline earth metal oxides are described as pigments in U.S. Pat. No. 2,661,336 to Lederer and U.S. Pat. No. 5,389,402 Speer et al., European Application No. EP 0 803 471 A2 to Degussa Aktiengesellschaft, and U.S. application Publication Ser. No. 2002/0,034,644 A1 by Swiler et al. The pigments are described as being colorants or polishing additives.
Rare earth sulfides have also been described as colorant pigments. Examples include European Patent No. EP 0 680 930 B1 to Rhodia Chimie; U.S. Pat. No. 6,221,473 B1 to Aubert et al.; and PCT International Application Nos. WO 00/00431 and WO 00/73210 A1 to Rhodia Chimie, and WO 01/42371 A1 and WO 01/74714 A1 to Rhodia Terres Rares.
Cerium and alkali- or alkaline-earth molybdates have been described as colorant pigments in PCT International Application No. WO 98/00367 to Rhone Poulenc Chimie. Cerium and alkali- or alkaline-earth titanates have also been described as colorants in U.S. Pat. No. 6,294,011 B1 to Hedouin et al. French Patent No. 2 785 896 to Rhodia Chimie describes the use of cerium (both trivalent and tetravalent) stannate as colorant pigments. Finally, U.S. Pat. No. 6,352,678 to Huguenin et al. describes the use of rare earth borates as luminophoric pigments.
Examples of trivalent cerium complexed with organic reagents include U.S. Pat. No. 5,167,709 to Shinohara, et al. British Patent No. 1,131,104 to Cooper et al., and British Patent No. 565,951 to Kvalnes et al. These pigments are described as colorants. Japanese Patent No. 9-188827 to Oshiumi et al. describes the formation of a colorant pigment formed by the action of cerium oxide or tetravalent cerium sulfate with organic dyes.
Japanese Patent No. 2-49075 to Yoneda, et al. describes the use of heteropoly acids containing cerium (among other metals) as being useful for antistatic coatings. PCT International Application No. WO 00/49098 to Rhodia Chimie describes the use of cerium oxide with organic liquids and ampophilic compounds to produce a sol which can be used as diesel fuel additives or for cosmetics.
Japanese Patent No. 6-101075 to Okuda et al. describes the use of trivalent cerium phosphate compound (among other phosphates) as an anticorrosive compound. Similarly, U.S. Pat. No. 5,006,588 to Miller describes the use of cerous (trivalent cerium) molybdate as a corrosion and crack growth inhibitor for use in coating and sealing compositions. U.S. Pat. No. 5,322,560 to DePue et al. describes the use of trivalent cerium acetate or molybdate as an anticorrosive agent for aluminum flake. The commonality between all three of these patent references is the use of trivalent cerium compounds. Trivalent cerium compounds utilized as pigments offer minimal corrosion protection due to lack of throwing power.
U.S. Pat. No. 6,096,139 to Shimakura et al. describes the use of triazinethiol derivatives in combination with metal ions as chrome-free anticorrosive coatings. The use of cerium is described (among other metals), but not the use of tetravalent cerium.
The use of colloidal suspensions of tetravalent cerium oxide (CeO2) in anticorrosive coatings is described in U.S. Pat. Nos. 5,922,330 and 5,733,361 to Chane-Ching et al.; and PCT International Application Nos. WO 96/26255 to Rhone Poulenc Chimie, and WO 01/36331 A1 and WO 01/38225 A1 to Rhodia Terres Rares. CeO2 exhibits a solubility that is too low for effective release of corrosion-inhibiting tetravalent cerium ions, implying that the anticorrosive effects of these colloidal suspensions is inferior to that described in the current art. Pigments are not typically used as aqueous colloidal suspensions.
U.S. application Ser. No. 432,610 filed Oct. 4, 1982 (see Nonferrous Metals, vol. 99, No. 56, p. 297 (1983) (Abstract)) describes the use of sodium dichromate, sodium tetraborate, sodium nitrite, sodium molybdate, ammonium hexanitratocerate, potassium hexachloropalladate, and lanthanum nitrate as crack-arresting compounds for high-strength alloys. These compounds are used in conjunction with methyl trialkyl (C8-C10) ammonium chloride and tricapryl (C6) methyl ammonium chloride as phase transfer catalysts for the organic phase. These compositions are not applied as pigments, but rather as hydrocarbon-washes.
U.S. Pat. No. 5,866,652 to Hager et al. describes the use of polymeric or sol-gel films in conjunction with mixtures of rare earth esters (such as acetates or oxalates), vanadates of alkali or alkaline earths, and alkali borates to provide corrosion protection. In other embodiments, the use of a rare earth chloride is described. However, Hager er al. do not describe cerium in the tetravalent oxidation state as being necessary to impart corrosion protection.
An aqueous dispersion of a cerium compound with other rare earth elements, transition metals, aluminum, gallium, or zirconium is described for anticorrosive agents in PCT International Application No. WO 01/55029 A1 to Rhodia Terres Rares. Similarly, an aqueous dispersion of cerium oxide in combination with additives such as beta-diketones, alpha-hydroxycarboxylic acids, beta-hydroxycarboxylic acids, or diols is described for anticorrosive agents in U.S. Pat. No. 6,033,677 to Cabane et al. Neither of these references define the need for cerium to be in the tetravalent oxidation state to achieve anticorrosive effects. Pigments are not typically used as aqueous colloidal suspensions.
U.S. Application Publication No. 2003/0024432 A1 by Chung et al. describes an anti-corrosive surface treatment comprising, inter alia, an organometallic compound that can include cerium (i.e., cerium acetate hydrate, cerium acetylacetonate hydrate, cerium 2-ethylhexanolate, i-propoxycerium, cerium stearate, and cerium nitrate). The disclosed coating is an anti-corrosive sol-gel that produces an adhesive film interface between a metal surface and an organic matrix resin or adhesive. In addition, U.S. Application Publication No. 2003/0019391 A1 by Kendig describes a corrosion inhibitor comprising an oxo-anion and a cation that is capable of inhibiting the propagation of pit corrosion on the surface of coated metal substrates, which coating can comprise a pigment. The cation can be a rare earth metal including cerium and praseodymium, inter alia. However, neither Kendig nor Chung et al. describe the need for cerium to be in the tetravalent oxidation state for corrosion inhibition.
U.S. Pat. No. 6,338,876 B1 to Ishii et al. discloses a process for hydrophilic treatment of an aluminum material comprising a primary step, wherein the primer contains the nitrate or related compound of a metal selected from aluminum, zirconium, cerium, chromium, and iron. The nitrate or related compound of a metal forms primer films that are corrosion-resistant, colorless, and transparent.
Finally, U.S. Pat. No. 6,428,846 B1 to Kaupp et al. discloses a process for producing a corrosion-stable aluminum-based metal pigment. In accordance with this process, a passivating protective layer is applied on the free metal surfaces of the pigment, which protective layer can comprise at least one of the following elements Al, Sn, Ti, V, Cr, Mo, Zn and Ce. However, tetravalent cerium is not described.
To date, no truly effective replacements have been developed for pigments based on CrVI. Accordingly, the need remains for improved corrosion-protective pigments composed of currently unregulated and/or nontoxic materials which have an effectiveness, ease of application, and performance comparable to current CrVI pigment formulations, and for methods of making and using the same.